Real-time ammonia emission monitoring during the composite propellant kneading

Abstract. The composite solid propellant (CSP) is generally manufactured with ammonium perchlorate (AP) and HTPB, using a bonding agent, typically TEPANOL. During the kneading process, the mixture promotes reactions between AP and the TEPANOL, generating ammonia (NH₃). However, NH₃ imprisonment inside the mixture leads to the reaction of ammonia with the isocyanate groups during curing, potentially compromising the mechanical integrity of the grain. Therefore, it is important to determine how much and for how long NH₃ is released during kneading. Thus, this work investigated the use of a portable ammonium detector to evaluate ammonia removal during mixing process of CSP under vacuum. The experimental setup involved the use of a portable Harvest E4000 electrochemical sensor positioned at the vacuum pump's exhaust during kneading. Monitoring started after AP addition and continued during vacuum mixing. Initial monitoring showed an increase in NH₃ levels during the early stages of manufacturing. In several instances, concentrations exceeded the sensor's limit, requiring the device to be moved further from the exhaust to maintain monitoring. As the process moved into final stages, such as the addition of coarser AP granulometries and aluminum, a general trend of reduction in ammonia concentration was observed. The data indicated that sensor positioning was critical for measurement stability. In tests where the sensor remained in a fixed position (inside the exhaust), the results showed a more uniform and clear decay behavior. For these optimized trials, ammonia levels stabilized near 0 ppm after approximately 240 minutes of mixing. The results demonstrate that the implementation of a portable sensor provides a viable, low-cost solution for the real-time monitoring of degassing efficiency. This monitoring approach can be utilized as a definitive indicator for the "end-of-process" by identifying when ammonia concentrations stabilize at 0 ppm.